Flash fire flammable cloud envelope calculator
A flash fire is the delayed ignition of a drifting flammable cloud that burns back through the cloud without generating significant overpressure. The hazard footprint is essentially the cloud itself, so the calculation is a dispersion calculation evaluated against flammability limits rather than toxicity endpoints.
Open the Flash Fire moduleWhat this calculator returns
- Downwind distance to LFL and to half-LFL
- Cloud width, height and flammable envelope
- Mass of fuel within the flammable range
- Burnt-area footprint for occupancy and escape assessment
Required inputs
- Release rate and duration from the source-term calculation
- Fuel LFL and UFL
- Atmospheric stability class and wind speed
- Surface roughness and release elevation
Calculation method
The cloud concentration field is produced by the dispersion model, neutral or dense depending on the released density.
Flammable contours are drawn at LFL and at half-LFL; half-LFL is the usual design contour because it allows for concentration fluctuations that a time-averaged model smooths out.
The flammable mass is integrated over the volume between LFL and UFL and passed to the explosion calculation when congestion is present.
Governing equations
C(x, y, z) from the dispersion solutionConcentration field used to locate the flammable contours.
x_LFL : C(x_LFL) = LFLDownwind distance at which the centreline concentration falls to the lower flammable limit.
M_flam = integral over V of rho C dV, LFL <= C <= UFLFuel mass inside the flammable envelope.
Nomenclature
- C
- concentration, vol fraction
- LFL, UFL
- lower and upper flammable limits, vol %
- x_LFL
- downwind distance to the LFL contour, m
- M_flam
- flammable mass in the cloud, kg
Assumptions and limitations
- Everyone inside the flammable cloud at ignition is assumed to be a fatality; outside it, radiation exposure is brief.
- The cloud is evaluated at its maximum extent unless a time-resolved assessment is requested.
- No significant overpressure is generated, which requires that the cloud is not in a congested region.
Reference practice
- Follows the flash fire treatment in the CCPS Guidelines for Consequence Analysis and standard QRA practice for occupied-area vulnerability.
Worked example
A 0.5 kg/s methane release under stability class D with 5 m/s wind, LFL 5 vol %.
| Step | Value | Basis |
|---|---|---|
| Dispersion case | Neutral, class D 5 m/s | Methane is buoyant, so a neutral or buoyant plume model applies |
| Distance to LFL | tens of metres | Set by where the centreline concentration decays to 5 vol % |
| Design contour | half-LFL, 2.5 vol % | Extends roughly 1.5 to 2 times further than the LFL contour |
| Footprint | Cloud envelope at ignition | Used directly as the flash fire fatality area |
The flash fire footprint is the cloud, so the whole calculation quality depends on the source term and the dispersion case selected, not on a separate fire model.
Illustrative numbers only — rerun the module with the project basis of design before using any result.
Common questions
Why is half-LFL used for flash fire footprints?
Dispersion models return time-averaged concentrations. Real clouds fluctuate, so pockets at LFL exist where the average is around half of it; using half-LFL accounts for that without a turbulence-resolving model.
How long does a flash fire last?
Seconds. The flame burns back through the cloud at a few metres per second, so exposure outside the cloud is short and the harm is essentially confined to the flammable envelope.
When does a flash fire become an explosion?
When the flammable cloud reaches a congested or confined region, flame acceleration generates overpressure and the event is modelled as a vapour cloud explosion instead.
Related calculators
- Gas Dispersion — Atmospheric gas dispersion calculator
- Vapour Cloud Explosion — Vapour cloud explosion overpressure calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases